With popularization of information terminals, there has been a growing demand for a flat panel display as a computer display. Moreover, with progress in information technology, information conventionally provided on a paper medium has been increasingly provided as electronic data. Electronic paper or digital paper has therefore been increasingly demanded as a thin, lightweight, easily portable mobile display medium.
In a flat panel display device, a display medium is generally formed by using an element utilizing liquid crystal, organic EL (organic electroluminescence), electrophoresis and the like. In such a display medium, a technology using an active driving element (TFT element) as an image driving element has been widely used in order to assure uniformity of screen brightness, a screen rewriting speed, and the like. In a typical computer display, for example, TFT elements are formed on a glass substrate and a liquid crystal, organic EL elements, or the like are sealed.
In this case, semiconductors such as a-Si (amorphous silicon) and p-Si (polysilicon) can be mainly used for the TFT elements. The TFT elements are manufactured by forming multiple layers of the Si semiconductors (and a metal film if necessary) and sequentially forming source, drain and gate electrodes on the substrate. Manufacturing of such TFT elements usually requires sputtering and other manufacturing processes of a vacuum system.
In manufacturing of such TFT elements, however, a manufacturing process of a vacuum system including a vacuum chamber needs to be repeated many times to form the layers, resulting in extremely high device cost and running cost. For example, processes such as vacuum deposition, doping, photolithography, and development usually need to be repeatedly performed to form the layers of the TFT elements, and the elements are formed on a substrate through several tens of processes. Even for a semiconductor portion that is critical for switching operation, a plurality of types of semiconductor layers such as p-type and n-type are stacked. In such a conventional Si semiconductor manufacturing method, drastic design change in a manufacturing apparatus such as a vacuum chamber is required for the needs for large-size display screens, and it is not easy to change facilities.
Moreover, formation of such conventional TFT elements using a Si material involves a high temperature process, adding a limitation on a substrate material that the substrate material needs to be resistant to the process temperature. Accordingly, glass needs to be used for practical applications. When a thin display such as electronic paper or digital paper as described above is formed by using the conventionally known TFT elements, the display is heavy, has poor flexibility, and may be broken if dropped. These characteristics resulting from forming the TFT elements on the glass substrate are not desirable to satisfy the needs for handy portable thin displays which have risen with progress in information technology.
On the other hand, organic semiconductor materials have been actively studied as organic compounds having high charge transportability. These compounds have been expected to be applied to organic laser oscillation elements and organic thin film transistor elements (organic TFT elements) in addition to a charge transportable material for organic EL elements.
If such organic semiconductor devices can be implemented, vacuum to low-pressure vapor deposition may be performed at a relatively low temperature, whereby the manufacturing process is simplified. Moreover, a semiconductor that can be brought into a solution may be obtained by appropriately improving the molecular structure. Therefore, manufacturing by using a printing method including an inkjet method may be implemented by using ink made of an organic semiconductor solution (Patent document 1 and the like).
Such manufacturing by the low temperature processes has been considered to be impossible for devices using the conventional Si semiconductor materials, but may be possible for devices using organic semiconductors. The above-described limitation regarding the heat resistance of the substrate is therefore reduced, and the TFT elements, for example, may be formed on a transparent resin substrate. If the TFT elements can be formed on the transparent resin substrate and a display material can be driven by the TFT elements, displays that are lighter and more flexible than the conventional displays and are not broken (or are unlikely to be broken) even if dropped can be implemented.
Patent document 1: Japanese Patent Laid-Open Publication No. 2007-67263
Patent document 2: Japanese Patent Laid-Open Publication No. 2006-186294